Lithium ion (Li-ion) batteries are currently the best performing batteries and already became the standard for portable electronic devices. In addition, these batteries already penetrated and rapidly gain ground in other industries such as automotive and electrical storage. Enabling advantages of such batteries are a high energy density combined with a good power performance.
A Li-ion battery typically contains a number of so-called Li-ion cells, which in turn contain a positive (cathode) electrode, a negative (anode) electrode and a separator which are immersed in an electrolyte. The most frequently used Li-ion cells for portable applications are developed using electrochemically active materials such as lithium cobalt oxide or lithium nickel manganese cobalt oxide for the cathode and a natural or artificial graphite for the anode.
It is known that one of the important limitative factors influencing a battery's performance and in particular battery's energy density is the active material in the anode. Therefore, to improve the energy density, newer electrochemically active materials based on e.g. tin, aluminium and silicon were investigated and developed during the last decades, such developments being mostly based on the principle of alloying said active material with Li during Li incorporation therein during use.
The best candidate seems to be silicon as theoretical capacities of 3579 mAh/g or 2200 mAh/cm3 can be obtained and these capacities are far larger than that of graphite (372 mAh/g) but also those of other candidates.
Note that throughout this document silicon is intended to mean the element Si in its zerovalent state. The term Si will be used to indicate the element Si regardless of its oxidation state, zerovalent or oxidised.
However, one drawback of using a silicon based electrochemically active material in an anode is its large volume expansion during charging, which is as high as 300% when the lithium ions are fully incorporated, e.g. by alloying or insertion, in the anode's active material—a process often called lithiation. The large volume expansion of the silicon based materials during Li incorporation may induce stresses in the silicon, which in turn could lead to a mechanical degradation of the silicon material.
Repeated periodically during charging and discharging of the Li-ion battery, the repetitive mechanical degradation of the silicon electrochemically active material may reduce the life of a battery to an unacceptable level.
In an attempt to alleviate the deleterious effects of the volume change of the silicon, many research studies showed that by reducing the size of the silicon material into submicron or nanosized silicon particles, typically with an average size smaller than 500 nm and preferably smaller than 150 nm, and using these as the electrochemically active material may prove a viable solution.
In order to accommodate the volume change, composite particles are usually used in which the silicon particles are mixed with a matrix material, usually a carbon based material, but possibly also a silicon based alloy or SiO2. In the present invention, only composites having carbon as matrix material are considered.
Further, a negative effect of silicon is that a thick SEI, a Solid-Electrolyte Interface, may be formed on the anode. An SEI is a complex reaction product of the electrolyte and lithium, and therefore leads to a loss of lithium availability for electrochemical reactions and therefore to a poor cycle performance, which is the capacity loss per charging-discharging cycle. A thick SEI may further increase the electrical resistance of a battery and thereby limit the achievable charging and discharging rates.
In principle the SEI formation is a self-terminating process that stops as soon as a ‘passivation layer’ has formed on the silicon surface. However, because of the volume expansion of silicon, both silicon and the SEI may be damaged during discharging (lithiation) and recharging (de-lithiation), thereby freeing new silicon surface and leading to a new onset of SEI formation.
In the art, the above lithiation/de-lithiation mechanism is generally quantified by a so-called coulombic efficiency, which is defined as a ratio (in % for a charge-discharge cycle) between the energy removed from a battery during discharge compared with the energy used during charging. Most work on silicon-based anode materials is therefore focused on improving said coulombic efficiency.
Current methods to make such silicon based composites are based on mixing the individual ingredients (e.g. silicon and carbon or a precursor for the intended matrix material) during preparation of the electrode paste formulation, or by a separate composite manufacturing step that is then carried out via dry milling/mixing of silicon and host material (possible followed by a firing step), or via wet milling/mixing of silicon and host material (followed by removal of the liquid medium and a possible firing step).
Despite the advances in the art of negative electrodes and electrochemically active materials contained therein, there is still a need for yet better electrodes that have the ability to further optimize the performance of Li-ion batteries. In particular, for most applications, negative electrodes having improved capacities and coulombic efficiencies are desirable.
Therefore, the invention concerns a composite powder for use in an anode of a lithium ion battery, whereby the particles of the composite powder comprise a carbon matrix material and silicon particles dispersed in this matrix material, whereby the composite powder further comprises silicon carbide whereby the ordered domain size of the silicon carbide, as determined by the Scherrer equation applied to the X-ray diffraction SiC peak having a maximum at 2θ between 35.4° and 35.8°, when measured with a copper anticathode producing Kα1 and Kα2 X-rays with a wavelength equal to 0.15418 nm, is at most 15 nm and preferably at most 9 nm and more preferably at most 7 nm.
The Scherrer equation (P. Scherrer; Göttinger Nachrichten 2, 98 (1918)) is a well known equation for calculating the size of ordered domains from X-Ray diffraction data. In order to avoid machine to machine variations, standardized samples can be used for calibration.
The composite powder according to the invention has a better cycle performance than traditional powders. Without being bound by theory, the inventors believe that the silicon carbide improves the mechanical bond between the silicon particles and the carbon matrix material, so that stresses on the interface between the silicon particles and the matrix material, e.g. those associated with expansion and contraction of the silicon during use of the battery, are less likely to lead to a disconnection of the silicon particles from the matrix material. This, in turn, allows for a better transfer of lithium ions from the matrix to the silicon and vice versa. Additionally, less silicon surface is then available for the formation of a SEI.
Preferably said silicon carbide is present on the surface of said silicon particles, so that said silicon carbide forms a partial or complete coating of said silicon particles and so that the interface between said silicon particles and said carbon is at least partly formed by the said silicon carbide.
It is noted that silicon carbide formation may also occur with the traditional materials, if silicon embedded in carbon or a carbon precursor is overheated, typically to well over 1000 degrees. However, this will in practice not lead to a limited, superficial formation of chemical Si—C bonds, as is shown to be beneficial in the present invention, but to a complete conversion of silicon to silicon carbide, leaving no silicon to act as anode active material. Also, in such circumstances a highly crystalline silicon carbide is formed.
The silicon carbide in a powder according to the present invention is present as a thin layer of very small silicon carbide crystals or poorly crystalline silicon carbide, which shows itself as having, on an X-Ray diffractogram of the composite powder, a peak having a maximum at 2θ between 35.4° and 35.8°, having a width at half the maximum height of more than 1.0°, which is equivalent to an ordered domain size of 9 nm as determined by the Scherrer equation applied to the SiC peak on the X-Ray diffractogram at 20=35.6°, when measured with a copper anticathode producing Kα1 and Kα2 X-rays with a wavelength equal to 0.15418 nm. Preferably, the composite powder has an oxygen content which is 3 wt % or less, and preferably 2 wt % or less. A low oxygen content is important to avoid too much lithium consumption during the first battery cycles.
Preferably the composite powder has a particle size distribution with d10, d50 and d90 values, whereby (d90−d10)/d50 is 3 or lower.
The d50 value is defined as diameter of a particle of the composite powder corresponding to 50 weight % cumulative undersize particle size distribution. In other words, if for example d50 is 12 μm, 50% of the total weight of particles in the tested sample are smaller than 12 μm. Analogously d10 and d90 are the particle sizes compared to which 10% respectively 90% of the total weight of particles is smaller.
A narrow PSD is of crucial importance since small particles, typically below 1 μm, result in a higher lithium consumption caused by electrolyte reactions. Excessively large particles on the other hand are detrimental for the final electrode swelling.
Preferably less than 25% by weight, and more preferably less than 20% by weight of all Si present in the composite powder is present in the form of silicon carbide, as Si present in the form of silicon carbide is not available as anode active material capable of being lithiated and delithiated.
In order to have an appreciable effect more than 0.5% by weight of all Si present in the composite powder should be present in the form of silicon carbide.
The invention further concerns a method of manufacturing a composite powder, preferably a composite powder as described above according the invention, comprising the following steps:                A: Providing a first product comprising one or more of products I, II and III        B: Providing a second product being carbon or being a precursor for carbon, and preferably being pitch, whereby said precursor can be thermally decomposed to carbon at a temperature less than a first temperature;        C: Mixing said first and second products to obtain a mixture;        D: Thermally treating said mixture at a temperature less than said first temperature;        whereby product I is: silicon particles having on at least part of their surface silicon carbide;        whereby product II is: silicon particles that can be provided on at least part of their surface with silicon carbide by being exposed to a temperature less than said first temperature and by being provided on their surface with a compound containing C atoms and capable of reacting with silicon at a temperature less than said first temperature to form silicon carbide; and        whereby product III is: silicon particles that can be provided on at least part of their surface with silicon carbide by being exposed to a temperature less than said first temperature and by being provided on their surface with a precursor compound for silicon carbide, said precursor compound comprising Si atoms and C atoms and being capable of being transformed into silicon carbide a temperature less than said first temperature;        whereby said first temperature is 1075° C. and preferably 1020°.        